I can't say for certain you can get any more detail without changing the canvas size unfortunately.
Yea, the canvas size is the resolution. Digitally speaking, you can not improve the quality of the image without increasing the canvas size. Realistically speaking, you don't even have to work as large as kurisil works.
Forewarning, I'm about to get carried away talking about the technical aspects of digital media. You don't have to read the whole thing, but it gives an explanation to why you can reasonably limit the size of your canvas.
Standard monitors now a days runs off of the 1.78:1 ratio (16:9) at a definition of 1080p. The "p" stands for "Progressive" -- compared to "i" which stands for "Interlaced" -- which I won't go into. The "1080" describes how many pixels are on the Y-Axis of your monitor / how many rows of pixels it has (i.e. it is 1080 pixels high).
This means that the size of the average monitor is 1,920 pixels wide and 1,080 pixels high. No matter how detailed you paint your image, this is all that you will be able to see at one time unless you zoom in. At this point you ask yourself how far people will typically be zoomed in. Typically, unless it's one of those panoramic pictures that you are supposed to zoom in and look around, you typically don't have to go higher than that.
Though....higher screen resolutions are starting to become a bit more common, so keep that in mind as well.
The goal location of where your art is going should be the sole thing you think about when deciding the document size. Unless there is a possibility it is going to print, you don't need more than 1,920 x 1,080 at 72 PPI until the average screen size changes.
It's true though; "When in doubt -- Go Big!" Also, if it's for personal use like a desktop background, click the "Print Screen" button on your Keyboard and that puts a screenshot of your monitor onto your clipboard. Then just open a new document from your clipboard and it should have all the proper information already there (Photoshop does this quite well, though I know you already said you're using Krita -- idk if it does it).